In recent years, from a viewpoint of preventing air pollution, an ammonia catalytic reduction method in which ammonia is used as a reduction agent to decompose nitrogen oxide (hereinafter, NOx) into nitrogen and water due to contact with a catalyst has been widely used as a method of removing NOx produced from a boiler or various burners. As the NOx removal catalyst, which has been currently put into practical use, a honeycomb catalyst having a rectangular hole shape has been the mainstream, in order to prevent blockages due to dust in flue gas and to widen a gas contact area. Furthermore, as a catalyst component, a type of catalyst component containing titanium oxide as a main component is known to be excellent, and another one containing vanadium, tungsten or the like as an active component is generally used. A binary TiO2—WO3 catalyst or a TiO2—MoO3 catalyst, and a ternary TiO2—V2O5—WO3 catalyst or a TiO2—V2O5—MoO3 catalyst are mainly used for the catalyst component.
In flue gas discharged from a coal-fired boiler, a calcium content contained in dust in flue gas mainly adheres to a surface of the catalyst to produce anhydrous calcium sulfate contained in flue gas and cover the surface of the catalyst, thereby blocking diffusion of NO and NH3 gas to the inside of the catalyst and deteriorating the performance of the catalyst. Conventionally, at the time of regenerating a catalyst, whose performance is deteriorated due to these causes of deterioration, washing the catalyst with water and an aqueous solution of hydrochloric acid has been known to be effective.
Furthermore, there has been proposed a technique for performing activation treatment of a catalyst with an acidic aqueous solution after reducing a substance accumulated on the catalyst by washing it with an aqueous alkaline solution, at the time of regenerating a NOx removal catalyst whose NOx removal performance deteriorates due to accumulation of arsenic (As2O3) present in flue gas, (Patent Literature 1).